V. Evaluation of Cross-Immunity against Type 1 Dengue Fever in Human Subjects Convalescent from Subclinical Natural Japanese Encephalitis Virus Infection and Vaccinated with 17D Strain Yellow Fever Vaccine
Between 1961 and 1968, 17 isolations of Silverwater virus were made from the blood of naturally infected snowshoe hares. Lepus americanus, and from all the life stages of Haemaphysalis leporis-palustris ticks collected from snowshoe hares captured in Alberta and Wisconsin. Each isolation was made in suckling mice, and identification was made by one or more of the following procedures: neutralization of infectivity, complement-fixation, or agar-gel double diffusion. Complement-fixation showed the isolates from Alberta and Wisconsin to be identical to each other and to the prototype strain of Silverwater virus. In experimental studies, viremia was detected in three of nine hares inoculated either intracranially or intravenously with 104 to 107 suckling mouse-ICLD50 of virus. Neutralizing antibodies, which neutralized at least 100 suckling mouse-ICLD50 of the homologous virus, were detected in the sera of seven hares about 30 days after exposure. No deaths or clinical signs of disease were observed in the hares or among the four other mammalian species inoculated, although viremia was detected in three species on the 5th day after inoculation. Cytopathogenic effect was observed in two of eight tissue culture systems inoculated with Silverwater virus.
Present address: College of Agriculture and Life Sciences, University of Wisconsin, Madison, Wisconsin 53706.
Present address: Sonoma State College, Cotati, California 94928.